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Engineering > Telecommunications trials study > Flashcards

Telecommunications trials study Flashcards

1
Q

3 Guided media types

A
  1. Twisted pair cable
  2. Coaxial
  3. Fibre optical
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2
Q

Twisted Pair Cables

A

Cheapest, shortest range, greatest EM/noise interference

Twisting reduces crosstalk
4 pairs of wire with different twist lengths

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3
Q

USB cables

A

Provides power and supports data transfer
- red and black for power
- green and white for signal

Coaxial Cable

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4
Q

Ethernet cables

A

Twisted pair cable

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5
Q

Coaxial Cable

A

Has 4 layers:
1. outer plastic sheath
2. Woven copper shield
3. Inner dielectric layer (insulator)
4. Copper core (or copper-clad steel)

Better noise resistance

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6
Q

Skin effect

A

AC power flows mainly along the surface
- skin depth dereases as frequeny increases
Coaxial cables opeerate at a

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7
Q

Copper-clad steel (CCS)

A

Increases skin depth

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8
Q

Fibre Optic Cables, pros and cons

A

Transmits light pulses
Several 100x faster the coaxial (up to 10km)

More immunity to noise, low signal attenutation, not susceptible to EMI, cannot be tapped (Security)
ALSO: digital signals, normally encoded

BUT fragile, expensive

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9
Q

Unguided media and types

A

NOT physically connected e.g.
radio (broadcast radio, TV)
microwave (directional transmissions e.g. mobile phone networks and satellites)
Infrared (remote controls, short range)

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10
Q

Directionality

A

Point-to-point: direct
vs
Broadcasting: various receivers

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11
Q

Mircophones/speakers

A
  1. Diaphragm vibrates when sound hits it
  2. Coil of wire with a magnet inside: Faraday’s law of induction
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12
Q

Antenna

A

Long piece of metal with a running current
- creates a magnetic field
- makes EM waves
- propagates in 3 dimensions

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13
Q

4 Types of radio waves

A

Direct: line-of-sight
Wave reflected from ground/ground waves: affected by terrain and vegetation (e.g. mountains): due to diffraction
Sky wave: skip propagation (bounces off ionosphere) - long distances only

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14
Q

Amplitude vs Digital signals

A

Analogues are infinitely variable (VERY complex), digitals have set levels (‘steps’) e.g. binary only has 1 or 0

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15
Q

Digitising steps

A
  1. Sampled at regular intervals
  2. This is quanitsed e.g. rounded to closest digital level (Quantisation)
  3. Encoded as a binary signal before transmission.

SQE: Stupid Quokkas Eat

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16
Q

Why digitise?

A

Security, can send more information in digital (channel capacity utilisation - multiplexing), noise immunity (ability to REGENERATE), lower cost for equipment

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17
Q

Bandwidth

A

Capacity for a channel to convey info (measured in bits for digital transmission)

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18
Q

Attenuation

A

Decrease in signal intensity (decibels dB)

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19
Q

Signal-to-noise ratio (SNR)

A

Higher means more resiliant

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20
Q

Regenerative repeaters

A

Take signal, then amplify: increases range
- first regenerates it to remove the noise
- ONLY for digital

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21
Q

Modulation, why it is important

A

Taking a message (baseband signal) and modifying a property so the message is easier transmitted.
Has high frequency carrier signal
MODEM: does the modulation

WHY?
1. Reduce antenna size
2. Reduce interference (send things at different frequencies)
3. Allow Multiplexing

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22
Q

3 Analogue Continuous wave modulation types (shift keying)

A
  1. Amplitude (very affected by noise, inefficient power use)
  2. Frequency (less noise interference as noise impacts amplitude, needs high bandwidth)
  3. Phase (ok for digital, not for analogue)
23
Q

LESS IMPORTANTL Pulse analogue modulation

A

Used for brightness of a lightbulb, etc.
- PAM: amplitude matches samples amplitude
- PWM: checks the width as well: Motor and LED control
PPM: position matches sampled

24
Q

Digital Modulation (transmission of signals in binary)

A

SHIFT KEYING
1. ASK: amplifude (high for 1, low for 0)
2. FSK: frequency (high for 1, low for 0)
3. PSK Phase (original is 1, shifted 180 out of phase is 0) - appears to flip

25
Q

Demodulation

A

Opposite of modulation: getting rid of carrier
1. Antenna is tuned to pick up a specific frequency (by coil and variable capacitor)
2. Diode rectifies the signal (makes positive)
3. Fixed capacitor smooths it
4. Earphones vibrates according to that -> converts to sound.

26
Q

Diode

A

Rectifies AC signal (only leaves positive)

27
Q

Guided media + pros and cons

A

Guided: transmission media that carries signals with a conductor e.g. fibre optic cables: has a PHYSICAL LINK
- short distance, but expensive for long distance/weird terrain

28
Q

Power Cables

A
  1. Low voltage - copper with PVC insulation
  2. High voltage - Cu or Al, shielded with polyethylene
  3. Extra high voltage - Al with steel core, not insulated.

(wire is often stranded to increase flexibility)

29
Q

Conductor Properies

A

Cu, Al Gold

High conductivity/low resistivity
High ductility
High Tensile strength
Good corrosion resistance
Low cost

30
Q

Copper propeties

A

High tensile strength
good conductivity
good ductility

31
Q

Pure copper types

A
32
Q

Why polymers?

A

Cheap
Ductile -> needs to be extruded
Low electrical conductivity (insulating)
Flexible

33
Q

Main polymers

A

Polyethylene
Polyvinyl Chloride (PVC)
Polypropylene
Nylon
ABS
Polycarbonate

34
Q

Polyethylene properties

A

retains insulation in humidity

BUT low softening temperature
Allows water vapour penetration
Relatively expensive

used for high-frequency interior cables (not used outside)

35
Q

PVC

A

Not as good an insulates and polyethylene, but tougher
higher softening temp

Naturally rigid by can be made flexible thru plasticisers

36
Q

Polypropelene

A

Silimar e properties to P.ethy but is tougher
highter softening temp

Harder: used in thin-wall insulation
but less flexible, expensive

37
Q

Nylon

A

Insect resistant outer jacket or sheath for underground use

Abrasion resistant
hard
smooth: difficult for insect/termite to grip

38
Q

ABS

A

Can be formed into complex shapes
tough and shock resistant
- commonly used in computer and phone casings (injection molding process)

39
Q

Polycarbonate

A

Similar mechanical properties to ABS
Good e insulation
heat resistant and flame retardant

Used in casings

40
Q

Testing techniques

A

Multimeter
Megger test
Oscilloscopes

41
Q

What needs to be tested (3)

A

Voltage
Current
Resistance

Test for inconsistencies and disruptions in circuit

42
Q

Multimeter

A

Combines several instuments in one e.g. voltmeter, anmeter. ohmmeter, etc

43
Q

Volt vs anm set up

A

Volt in parallel, anmeter in series

44
Q

Insulation testing why

A

PRevent e shoks
ensure safety
reduct equiptment downtime

45
Q

Megger testing

A

test for insulation faults: megohmmeter applied high voltage, measures resistance over time

high resistance = good, low = fault

46
Q

Oscilloscope

A

Measure how signal changes over time
Can show:
Voltage/current changes over time
frequency of oscillating signal
Malfunctioning components
How much of signal is DC and AC
How much noise, whether noise is changing

47
Q

Frequency Analysis

A

using a spectrum analyser
- use Fast Fourier Transform (FFT) to covert signals from time domain to frequency domain

48
Q

Multimode cables

A

Short distance
High bandwidth support
higher cable cost
Lower electronics cost
Easier to terminate due to large core size

49
Q

Single mode cable

A

Short and long distance
Highest bandwidth support
Lower cable cost
Higher electromics cost
Harder to terminate with smaller core size

50
Q

Structure of Fibre OC

A
  1. high refractive index core
  2. Low r.i. cladding (ensures light signals stay trapped in core even with bending)
51
Q

Advantages of FO

A

Lighter
Corrosion resistant
Wide band width + high transmission capacity (light faster then electricity

52
Q

Problems of Fo

A

Attenuation: atomic absorption of light by glass
Scattering of light by flaws and impurities
Reflection of light by splices and connectors

DISPERSION:
Spreading/overlapping of light pulses with distance
caused by chromatic dispersion (different wavelengths, pink Floyd)
and modal dispersion
- fibres w large diameters: light travel along different modes/paths
- limits bit rate.

53
Q

Multimode fibre types

A

allows alight to travel along many modes/paths

  1. Step index: high R.I core and low R.I cladding
    - high attenuation and dispersion
  2. Graded index: cores with higher R.I in the middle
    - light travelling straight down travels slowly: reduces dispersion
54
Q
A

Engineering (10 decks)

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